I posted a spreadsheet that I rediscovered I had made a while back. It will tell you what gears you should switch to to get the factory performance back. You just need the old and new tire sizes, gear ratio and RPMs at 55mph.

To view it you need IE4.01+ and office 2000 with the pivot table installed:
http://members.home.net/mjbobbitt/f150/tirecalc.html

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1997 XLT: 4x4, 4.6 5 sp manual. supercab, 3.55LS. Eagle Series 171 16x8 wheels with 285/75/16 BFG ATs.

1990 Mustang GT. Black, 5sp. Usual mods, about 250bhp.

 

thanks

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Dr.(wife says i'm spoiled) Jones.
2000 F-150 5.4L(whatpowerawaitsthee)
AUTO,SC-sb 4x4-XLT, 3.55LS, 8.8 front- 9.75 rear, sidesteps, sprayed in liner, 17"wheels ORP, trailer-tow pkg, CD, Cpt chairs, 4whl ABS, Keyless Entry, Airbox silencer removed/K&N, Amsoil(transfercase, motor, rearend), 3" custom dynomax ultra flow catback, as an anniversary gift. PSD coming for christmas!
djones01@elp.rr.com

 

Matt,

Great post, but I have a question: When measuring the diameter of the tire where exactly do we measure? Thanks in advance..

 

I just go to any tire manufacturer's website and look up the specs of the tire.